I. Field of the Invention
The present invention relates generally to the field of catheter-based RF tissue ablation devices and techniques and, more particularly, to catheter devices and methods for assessing the efficiency of electrode-tissue contact related to the effectiveness of tissue ablation to relieve atrial cardiac arrhythmias. Specifically, the invention provides a device and method for determining ablation electrode to tissue contact before and during an ablation procedure and the estimation of tissue heating during RF application.
II. Related Art
Cardiac arrhythmias, particularly atrial fibrillation, are common and dangerous medical conditions causing abnormal, erratic cardiac function. Atrial fibrillation is observed particularly in elderly patients and results from abnormal conduction and automaticity in regions of cardiac tissue. Chronic atrial fibrillation (AF) may lead to serious conditions including stroke, heart failure, fatigue and palpitations. The treatment of chronic AF requires the creation of a number of transmural contiguous linear lesions. The use of a pattern of surgical incisions and thus surgical scars to block abnormal electrical circuits, and passageways known as the Cox Maze procedure, has become the standard surgical procedure for effective surgical cure of AF. The procedure requires a series of full-thickness incisions to isolate the pulmonary veins and the posterior wall of the left atria. Additional lines involve the creation of lesions from the posterior wall to the mitral valve, at the atrial isthmus line and superior vena cava (SVC) to the inferior vena cava (IVC) with a connection to the right atrial appendage.
Catheters have been developed that make the corrective procedure less invasive. They are designed to create lesions by ablation of tissue that perform the function of the surgical incisions. These include catheters that attempt to connect a series of local or spot lesions made using single electrodes into linear lesions. Devices that use a linear array of spaced electrodes or electrodes that extend along the length of a catheter have also been used.
Important drawbacks found fundamental in the current catheter-based ablation approaches can be attributed to several factors including a lack of consistent contact between the ablation devices and the target tissues, and the inability to accurately determine the degree of ablation electrode contact with the targeted tissue prior to and during the ablation procedure.
Effective RF tissue ablation is a function of the ablation electrode contact with the targeted tissue and the current density that result in tissue heating and tissue destruction. The most effective lesion is created if the ablation electrode is imbedded in the tissue. Clearly, the assumption that the application of increasing force increases the tissue surface contact with the ablation tip resulting in an effective lesion. However, in a thick, stiff tissue, such as the AV junction, increasing force may translate to only a small improvement in contact area, whereas in thin pliable tissues, such as the PV/LA junction, the same contact force results in the catheter creating a pouch that is likely to lead to extra-cardiac injuries and possible cardiac perforation. Several technologies are currently in clinical use measuring the ablation catheter contact force. These technologies utilizing laser and electromagnetic sensors do not provide any information regarding the ablation electrode to tissue contact and also are costly. Another approach that is also used is an algorithm assessment of contact based on impedance change associated with the impedance differences of blood vs. tissue.
Additional limitation of the current ablation technologies is the lack of information regarding the tissue temperature. To avoid char formation from overheating the tissue that can lead to stroke most if not all ablation procedures applied in the left atria employ open irrigated catheters. The irrigation flow cools the ablation electrode and removes/dilutes the blood trapped between the electrode and the tissues preventing char formation during the RF application. As a result of the cooling by the irrigation the temperature measurement of RF tissue heating is not possible. The proposed technology will allow not only assessment of the catheter-to-tissue contact but also an assessment of tissue heating during the RF application.